Persistent nodes for RFID

ABSTRACT

An RFID transponder in one embodiment comprises a radio frequency (RF) transceiver, processing logic coupled to the RF transceiver, a switch coupled to the processing logic, a tunneling device coupled to the switch and a differential sensing circuit having a first input coupled to the tunneling device and a second input coupled to a predetermined reference voltage. In one embodiment, the tunneling device can discharge to a voltage below the predetermined reference voltage.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of radio frequencyidentification (RFID) devices or tags and specifically to RFID deviceswhich include one or more persistent nodes.

RFID transponders (commonly referred to herein as “tags”) in the form oflabels, inlays, straps or other forms are widely used to associate anobject with an identification code. Tags generally include one or moreantennas with analog and/or digital electronic circuits that includecommunications electronics (such as an RF transceiver), data memory (forstoring one or more identification codes), processing logic (such as amicrocontroller) and one or more state storage devices. Examples ofapplications that can use RFID tags include luggage tracking, inventorycontrol or tracking (such as in a warehouse), parcel tracking, accesscontrol to buildings or vehicles, etc.

There are three basic types of RFID tags. A passive tag is a beampowered device which rectifies energy required for operation from radiowaves generated by a reader. For communication, the passive tag createsa change in reflectivity of the field which is reflected to and read bythe reader. This is commonly referred to as continuous wavebackscattering. A battery-powered semi-passive tag also receives andreflects radio waves from the reader; however a battery powers the tagindependent of receiving power from the reader. An active tag, having anindependent power supply, includes its own radio frequency source fortransmission.

The reader, sometimes referred to as an interrogator, includes atransmitter to transmit RF signals to the tag and a receiver to receivetag modulated information. The transmitter and receiver can be combinedas a transceiver which can use one or more antennas. Communicationsbetween a reader and tag is defined by an air interface protocol, suchas (without limitation):

(i) EPCglobal's EPC Radio-Frequency Identity Protocols Class-1Generation-2 UHF RFID Protocol for Communications at 860 MHz-960 MHz,version 1.2.0 (http://www.epcglobalinc.org/) (hereinafter referred to asthe “UHF Gen2 standard”);

(ii) adaptations of the UHF Gen2 standard for operation at highfrequency (“HF”), for example at 13.56 MHz; and

(iii) ISO/IEC 18000-6 Information technology—Radio frequencyidentification for item management—Part 6: Parameters for air interfacecommunications at 860 MHz to 960 MHz, Amendment 1: Extension with Type Cand update of Types A and B. Each of the above protocols is incorporatedherein by reference for all purposes.

Communication protocols, such as these, may require that a passive tagoperate a timing circuit or maintain a flag value during a brief lapseof received power which can occur when a reader hops betweentransmission frequencies. For example, the UHF Gen2 standard requirespersistence for flags SL, S1, S2, and S3, but not S0. U.S. Pat. No.6,942,155 and pending U.S. application Ser. No. 12/420,009, filed Apr.7, 2009, both assigned to Alien Technology Corporation (“Alien,” alsothe assignee to this invention) and incorporated by reference herein forall purposes, provide various teachings on persistent flags and nodes.Other or related techniques have been suggested by the following patents(each of which is incorporated by reference herein for all purposes):U.S. Pat. No. 7,259,654; U.S. Pat. No. 7,710,798; and U.S. Pat. No.7,215,251.

It should be clear from the teachings herein that a persistent flag is abit, character(s), or other indicator that signals the occurrence ofsome condition. The persistent flag can be stored in a persistent nodethat provides a state storage device. The persistent node is a circuitwhich is initialized to a value, and the value read from the persistentnode can change at some later time. Persistent flags can be implementedusing persistent nodes as described in one or more of the incorporatedreferences. As an example, persistent flags can be implementedessentially as a timer using persistent nodes. For example in theISO/IEC 18000-6c specification, each flag has one of two values. “A” or“B” for the S1, S2 or S3 flags, and “asserted” or “deasserted” for theSL flag.

Passive RFID tags can lose power whenever a reader is turned off for aperiod of time that is longer than the tag can support supplying currentfrom its power capacitor(s). Currently known methods of implementing astate storage bit or flag in a state storage device include the use ofan FET (Field Effect Transistor) to charge/discharge a capacitor so thatthe leakage through the FET in the off state determines the dischargetime for the state storage bit. Since the high impedance of the FET (inthe off state) depends on parasitics, when the power supply is off(e.g., the reader stops transmitting), the state storage devicedissipates its charge by means of an unknown and widely varying leakagecurrent. As a result, these implementations can cause the capacitor todrain current too quickly or allow the capacitor to retain a charge fortoo long. Hence, in these implementations, the state storage time canvary with ambient temperature (e.g. tags in a cold warehouse vs. tags ina hot warehouse will have different state storage times) and can varydue to processing variations (from variations in processing operationsin the semiconductor wafer and IC fabrication process), and thisvariation can be from a few seconds to a few hours. A known method ofreducing the variation of the current discharging device is the use of acalibration method to keep a FET transistor gate bias blocking thedischarge of the capacitor at a voltage which gives a substantiallyconstant current; another known method to reduce this variation is totrim the devices to minimize process variations. These known methodseither result in significant variation in the timing of the circuit orsubstantial additional cost due to additional semiconductor ICfabrication processing or additional circuit area to provide calibrationcircuits.

FIG. 1 shows an example in the prior art which uses a thin oxidecapacitor as a state storage device for an RFID tag. The state storagedevice 11 in FIG. 1 includes an n-channel FET 12 having its draincoupled to a supply voltage V_(dd) (or to another charging ordischarging node which supplies a voltage derived from a voltage source)and its gate 14 coupled to processing logic (not shown) to either chargeor not charge the capacitors 15 and 16 which are coupled, at node 19, tothe source of FET 12. The capacitors 15 and 16 are also coupled toV_(ss) (which can be ground). The FET 12 acts as a switch, which iscontrolled by the signal applied to gate 14, to either charge or notcharge the capacitors 15 and 16. Capacitor 16 is a thin gate oxidecapacitor which supplies the majority of the capacitance due to its thindielectric, and also allows current through the dielectric due totunneling; the capacitor 16 is disposed in the substrate of asemiconductor IC (integrated circuit) that includes the state storagedevice 11. Capacitor 15 is a capacitor fabricated in the metal andinsulator layers above the substrate and hence capacitor 15 is referredto as a metal-insulator-metal (MIM) capacitor. The capacitance ofcapacitor 16 exceeds the capacitance of capacitor 15. Node 19 can beconsidered the output of the state of the state storage device 11, andthis output is coupled to one input of a balanced sense amp 18 that alsoreceives an input from a set of capacitors 15A and 16A that arefabricated to match identically capacitors 15 and 16. Capacitors 15A and16A remain at a fully discharged state, and the balanced sense amp 18determines whether the output at node 19 exceeds the fully dischargedstate of capacitors 15A and 16A. The balanced sense amp 18 isimplemented as a current mirroring circuit that flips an output one wayor the other way depending upon whether the node 19 exceeds the fullydischarged state of capacitors 15A and 16A. The balanced sense ampallows discharge to a very low level, and the time to discharge of thenode depends on the total capacitance of capacitors—15 and 16, thesemiconductor leakage through the FET 12 at high temperatures, and theleakage through the oxide of cap 16 at low temperatures. The time todischarge varies from below 1 second at high temperatures, limited byleakage through FET 12, and over 120 seconds at low temperatures,limited by the leakage of cap 16. The process variation is alsoapproximately a factor of 5, due to the discharge to very low level,maintaining an adequate persistent node duration.

SUMMARY OF THE DESCRIPTION

In one embodiment, a state storage device of the present inventionprovides a persistent node with good behavior over semiconductorfabrication process variations and over changes in temperature, and thisgood behavior is obtained at a lower cost due to the smaller size of thecircuit and without any expensive trimming as in the prior art.

In one embodiment, an RFID tag can include a state storage device whichcomprises a switch coupled to processing logic and a tunneling devicecoupled to the switch and to a differential sensing circuit which has afirst input coupled to the tunneling device and a second input coupledto a predetermined reference voltage. The discharge time of thetunneling device is set to be determined by a tunneling current. Anexample tunneling device useful for this purpose is a thin silicondioxide layer with a thickness between 10 angstroms and 50 angstroms. Anexample of a tunneling device is a gate oxide layer of a low voltageCMOS process. The gate oxide layer is one of the most tightlyconstrained and controlled parameters in semiconductor fabricationprocessing and hence the tunneling current can be accurately controlledusing current semiconductor fabrication processing techniques. Thetunneling current is not substantially constant but varies with thevoltage on the structure. Modeling CMOS Tunneling Currents ThroughUltrathin Gate Oxide Due to Conduction- and Valence-Band Electron andHole Tunneling Wen-Chin Lee, Member, IEEE, and Chenming Hu, Fellow,IEEE. IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 7, JULY 2001.

The discharge time of the tunneling device can be designed by selectingthe size of the capacitor, the size of the tunneling device, thestarting voltage and the terminal voltage which can be the predeterminedreference voltage. The switch leakage current is made negligible incomparison to the tunneling current by the use of a very low leakageswitch such as a very long gate MOS device. Since the tunneling currentcan be relatively independent of ambient temperature and since the oxidethickness or gate oxide thickness is generally tightly controlled, thisembodiment results in a high performing, relatively inexpensive solutionfor a persistent node or state storage device which can continue timingwith or without power being provided to the RFID transponder. In oneembodiment, the predetermined reference voltage can be set to a valuewhich is above the fully discharged voltage of the tunneling device byreducing the discharge current to an extremely low but predictablecurrent provided by the tunneling device, and the predeterminedreference voltage can be provided by a reference voltage generator whichdoes not need to be very low, not needing to be measured against abalanced device, and that does not require include any tunnelingdevices. The switch, in one embodiment, can be an FET made with achannel long enough to make the leakage current through the FETnegligible.

An RFID transponder, in one embodiment, can include an RF (RadioFrequency) transceiver which includes both a transmitter and a receivercoupled to one or more antennas, and processing logic coupled to the RFtransceiver, and a first switch coupled to a first reference voltage,and a capacitor coupled to the switch and a tunneling device coupled tothe switch and coupled in parallel with the capacitor. The RFIDtransponder also includes a differential sensing circuit coupled, at afirst node, to the capacitor and to the tunneling device. The first nodeis a charge storage node. The differential sensing circuit determineswhether a voltage at the first node is above a predetermined referencevoltage which can be generated by a reference voltage generator that inone embodiment does not include any tunneling devices. Moreover, thepredetermined reference voltage can be above the fully dischargedvoltage of the tunneling device capacitor in the state storage device.The differential sensing circuit indicates a first state when thevoltage at the first node is above the predetermined reference voltageand indicates a second state when the voltage at the first node is belowthe predetermined reference voltage. The switch can be coupled to theprocessing logic in order to determine when and whether the capacitorsare charged in a manner which is consistent with the known protocolssuch as the EPC protocol or the ISO/IEC specification referred toherein.

The tunneling device can include a thin gate oxide which separates afirst gate of the tunneling device from a first substrate region in asubstrate of a semiconductor integrated circuit. The capacitor can alsobe disposed in the substrate and include a thick gate oxide whichseparates a second gate from a second substrate region in the substrateof the semiconductor integrated circuit, wherein the second gate and thesecond substrate region act as plates of the capacitor. The tunnelingdevice and the capacitor can discharge through a range of voltages thatis defined by the first reference voltage at one end of the range and bythe predetermined reference voltage, which can be above a ground voltageand the tunneling device and the capacitor can continue to discharge tovoltages below the predetermined reference voltage.

The predetermined reference voltage can be generated by a referencevoltage generator which is coupled to the first reference voltage and toa ground voltage and which is coupled to the sensing circuit; in oneembodiment, the reference voltage generator does not include anytunneling capacitors and hence generates the predetermined referencevoltage independently of the operation of a tunneling capacitor. In oneembodiment, the capacitor has a substantially greater area than thetunneling device. In one embodiment, the capacitor has a substantiallygreater capacitance than the tunneling device. In one embodiment, thetunneling device is a thin oxide capacitor which has a substantiallysmaller area than the capacitor. In one embodiment, the tunneling deviceis a thin oxide capacitor which has a substantially smaller capacitancethan the capacitor. The switch can be coupled to the processing logicand can include a field effect transistor which selectively charges thecapacitor and the tunneling device. In one embodiment, the majority ofthe discharge of the capacitor is through a tunneling current throughthe thin gate oxide and the field effect transistor in the switch has along channel length to make the leakage current through the field effecttransistor negligible over the operating temperature range. In oneembodiment, the operating temperature range is from −25 to +40 degreesCelsius. In one embodiment, the operating temperature range is from −25to +85 degrees Celsius.

The state of charge of the capacitance of the current invention forms atimer which continues to work during a loss of power in the RFIDtransponder. The timer can store a flag state for a predetermined periodof time which can be substantially independent of ambient temperature.For example, the predetermined period of time provided by the inventioncan be substantially independent of changes in temperature from −25 to+40° C. The predetermined period of time can be more than 0.5 secondsand less than 120 seconds in one embodiment, and in another embodimentthe predetermined period of time is more than 0.5 seconds and less than20 seconds. The predetermined period of time can be more than 0.5seconds and less than 5 seconds in one embodiment. In one embodiment,the RFID transponder can include a dipole antenna coupled to the RFtransceiver, and this dipole antenna can be configured to receive an RFsignal from a reader and to backscatter a responsive RF signal to thereader. In one embodiment, the first reference voltage (which is coupledto the switch) can be variable in order to vary the period of time forwhich the state of a flag is to be held high. In another embodiment, thepredetermined reference voltage can be variable in order to vary thepredetermined period of time for which the state of the flag is topersist. In yet another embodiment, both the first reference voltage andthe predetermined reference voltage can be variable in order to vary thepredetermined period of time. The flag state to be held for a selectedtime can be a timeout interval, marking the time since the flag was setin an inventory, since a password attempt was made, etc. The time toexpiration of the flag may also depend on an externally provided voltageor stimulus such as incident light.

In one embodiment, multiple persistent nodes may be implemented forvarious functions of the tag, including the Session flags of ISO18000-6c or the SL flag of that same protocol. In one embodiment, theflags may only discharge during periods when no power is supplied to thetag, and refreshed when power becomes available only if they have notalready expired. In one embodiment, there may be nodes which time thetimeout period for security protocols which require an interval of timeto pass before another password attempt or other security procedure isallowed.

In one embodiment, the RFID transponder can include an optionaldischarge circuit which is coupled to the processing logic and which iscoupled to the charge storage node in order to selectively discharge thecharge storage node in response to a signal from the processing logic.

In one embodiment, the discharging of the capacitors in the capacitivecircuit of the state storage device, can be dominated by the tunnelingcurrent when the state storage circuit's temperature is above about 40°C.

The above summary does not include an exhaustive list of all aspects ofthe present invention. It is contemplated that the invention includesall systems and methods that can be practiced from all suitablecombinations of the various aspects summarized above, and also thosedisclosed in the Detailed Description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings in which likereferences indicate similar elements.

FIG. 1 shows an example of a state storage device in the prior art.

FIG. 2 shows a block diagram of an RFID transponder according to oneembodiment of the present invention.

FIG. 3 shows an example of an RFID reader which can be used with an RFIDtransponder described herein.

FIG. 4A is a block diagram schematic of a state storage device for usein an RFID transponder according to one embodiment of the presentinvention.

FIG. 4B is a block diagram schematic of a state storage device which canbe used in an RFID transponder according to an embodiment of the presentinvention.

FIG. 5A is a circuit schematic showing an example of a state storagedevice according to one embodiment of the present invention which can beused in an RFID transponder.

FIG. 5B is an example of a reference voltage generator which can providea reference voltage, such as V_(ref) according to one embodiment of thepresent invention.

FIG. 5C is an example of a reference voltage generator which can providedifferent reference voltages.

FIG. 6 is a voltage vs. time graph showing the discharging of a chargestorage node over time and showing the relationship of a predeterminedreference voltage relative to the discharge curve in the graph of FIG.6.

FIG. 7 shows a cross-sectional view through a semiconductor substrate,wherein the view shows two capacitors disposed at least partially inthat substrate according to one embodiment of the present invention.

DETAILED DESCRIPTION

Various embodiments and aspects of the inventions will be described withreference to details discussed below, and the accompanying drawings willillustrate the various embodiments. The following description anddrawings are illustrative of the invention and are not to be construedas limiting the invention. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentinvention. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present inventions.

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin conjunction with the embodiment can be included in at least oneembodiment of the invention. The appearances of the phrase “in oneembodiment” in various places in the specification do not necessarilyall refer to the same embodiment. The processes depicted in the figuresthat follow are performed by processing logic that comprises hardware(e.g. circuitry, dedicated logic, etc.), software, or a combination ofboth. The processing logic may consist of a finite state machine, orseveral interconnected finite state machines. Although the processes aredescribed below in terms of some sequential operations, it should beappreciated that some of the operations described may be performed in adifferent order. Moreover, some operations may be performed in parallelrather than sequentially.

The state storage device of the present invention can be used to storedata such as a bit in a volatile memory cell, and the stored bit can beused as a flag as is known in the art that can be retained over shortperiods of time when the RFID transponder or tag loses power, such aswhen a passive RFID transponder no longer receives a signal from areader which provides power to the transponder. In some embodiments, thestate can be passively stored on one or more capacitors as is describedherein. When the tag or transponder loses power, the capacitor canretain its charge over a period of time as it slowly discharges. Thus,when the reader provides power again, the tag can recognize its storagestate or flag and operate accordingly based upon the protocol being usedby the tag, such as the EPC global protocol referred to herein or theISO/IEC 18000-6 specification referred to above. In some embodiments,the state storage device can be set to provide a storage time which is apredetermined or otherwise known time selected to avoid missing tags insubsequent interrogation rounds due to a missed command to reset thestate storage bit or for other reasons to provide a known time. Thisknown time can provide a known upper limit on the storage duration ofthe bit stored in a state storage device. As is known in the art, alimited retention time in the state storage device helps prevent tagsthat have already been identified from entering the protocolidentification process or other protocol repeat process again, whileallowing tags which have not been inventoried for a period of time tore-enter the protocol identification process, and thereby increasesthroughput in the process of inventorying or counting or otherwiseidentifying RFID tags or transponders. The time period can start whenthe flag is set, and then a process can check the status of the flag ata later point in time, or the discharge may only start when an intervalwhen the tag has no power from the reader, and be considered timed outonly if it discharges below the predetermined reference voltage beforepower is again available.

FIG. 2 provides a block level representation of an RFID transponderaccording to one embodiment of the present invention. The tag ortransponder 201 includes one or more antennas, such as antenna 211which, in one embodiment, can be a dipole antenna, a t-match meanderedantenna with end loading, or a loop antenna or other antennas known inthe art. The antenna 211 is coupled to an RF transceiver 203 whichprovides a radio circuit including both a transmitter and a receiver.The receiver receives signals from an RFID reader, such as the RFIDreader shown in FIG. 3 and the transmitter of the transceiver 203transmits signals to an RFID reader, such as the RFID reader shown inFIG. 3. The RF transceiver 203 is coupled to processing logic 205 whichcan be implemented in a variety of different ways, including amicrocontroller or a programmable logic device, or an ASIC controlcircuitry, etc. Processing logic 205 is coupled to the RFID memory 209and is coupled to the state storage device 207. Tag 201 can include oneor more state storage devices 207 to store one or more states, eachhaving at least one bit for a particular state. In one embodiment, thetag 201 can include four state storage devices 207, each receiving aseparate control signal to control its respective switch, the controlsignals being provided by processing logic 205 as is known in the art.Examples of circuits which can implement the state storage devices 207are provided in FIGS. 4A, 4B, and 5A and are further described below.RFID memory 209 can be non-volatile memory such as a mask read-onlymemory (ROM), electrically erasable (EE) programmable read only memory,or flash memory or other non-volatile memory which can store informationfor the transponder, such as the tag's identification number oridentification code and other information as is known in the art.Processing logic 205 can retrieve the data from RFID memory 209according to the various protocols under which the tag can operate andcan transmit those identification values to a reader through the RFtransceiver 203 as is known in the art.

FIG. 3 shows an example of an RFID reader which can operate with any oneof the RFID transponders described herein. For example, the reader shownin FIG. 3 can operate with the tag 201 which can include the statestorage device 207 implemented as shown in any one of FIGS. 4A, 4B, and5A. Reader 301 can include one or more antennas, such as antenna 303,which is coupled to an RF transceiver 305 as is known in the art. The RFtransceiver 305 can be coupled to a processing system 307 which in turnis coupled to memory 309 and to input/output interfaces 311 as is knownin the art. The input/output interfaces 311 can provide an interface toother systems such as computers or other devices which are coupled tothe reader 301 in order to receive data from the tags queried by thereader 301. The RF transceiver 305 can operate in conjunction withprocessing system 307 as is known in the art to implement any one of theknown protocols for communicating with RFID tags such as the EPC globalprotocol referred to above or the ISO/IEC protocol referred to above.

FIG. 4A shows a block diagram of a state storage device according to oneembodiment of the present invention. The state storage device caninclude a switch 404 which receives a first reference voltage 412 whichcan be, in one embodiment, a power supply voltage such as V_(dd) or avoltage derived from a voltage source. In one embodiment, the firstreference voltage 412 acts as a charging node to charge the capacitancecircuit. In one embodiment, the first reference voltage 412 acts as acharging node to charge the capacitance circuit and at different pointin time, the first reference voltage 412 acts as a discharging node todischarge the capacitance circuit. The switch 404 in the state storagedevice 401 includes an input to receive a state input signal 416 whichis used to control the state of the bit or flag stored by the statestorage device 401 as is known in the art. The switch 404 in the statestorage device 401 is coupled to the node 408, and node 408 is alsocoupled to sense amp 406 and to an optional discharge circuit 410 whichis controlled by a control signal 418 which can be provided byprocessing logic to selectively discharge the capacitance circuit 402 inresponse to a command from the control signal 418. The node 408 can beconsidered a charge storage node. The state input signal 416 can beprovided by processing logic, such as processing logic 205 as is knownin the art, and processing logic 205 can also provide the control signal418 to the optional discharge circuit 410 to cause the capacitancecircuit 402 to discharge. Capacitance circuit 402 is coupled to node 408and is also coupled to a second reference voltage 414 which can be apower supply voltage such as V_(ss) in one embodiment. The capacitancecircuit 402 can include one or more capacitors, each implemented anddisposed at least partially in a substrate of a semiconductor integratedcircuit. In one embodiment the capacitance circuit can include both athick oxide capacitor and a thin oxide capacitor coupled together inparallel. In one embodiment, the capacitance circuit can include acapacitor disposed in either the substrate or above the substrate as ametal-insulator-metal capacitor. In one embodiment, the tunneling devicecan include a thin dielectric which is not an oxide, such as siliconnitride, or other dielectrics.

State storage device 401 can be operated in a manner which is consistentwith the protocols known in the art, such as the EPC global protocol orthe ISO/IEC protocol referred to above. For example, the processinglogic can cause the switch 404 to charge node 408 to within a thresholdvoltage of the first reference voltage 412 which in turn will charge thecapacitors within the capacitance circuit 402. Then the processing logiccan turn off the state input signal 416 to shut off the switch and tothereby isolate the node 408 from the first reference voltage 412 toallow the capacitors within the capacitance circuit 412 to retain acharge even if power is lost in the tag (such as when the RFID readerstops transmitting a signal to the passive RFID tag which contains thestate storage device 401). The voltage node 408 discharges over timeafter a loss of power and at a later point in time when the tag isreceiving power, the sense amp 406 can determine the state of node 408to determine whether or not the capacitance circuit has been dischargedand thereby determine the state of the flag or bit stored by the statestorage device 401.

FIG. 4B shows an example of an implementation of the circuit shown inFIG. 4A in which the sense amp 406 is implemented as a differentialsensing amplifier which includes two inputs, one of which receives thevoltage from node 408 and is coupled to node 408 as shown in FIG. 4B,and the other of which receives a reference voltage 409. Thedifferential sense amp 406A provides an output 407 which indicates thestate of the state storage device 401A based upon the comparison betweenthe voltage at node 408 and the reference voltage 409 which can be apredetermined reference voltage as in the examples provided below.

FIG. 5A shows a circuit schematic of a state storage device 401B whichis similar to state storage device 401A of FIG. 4B. Switch 404 has beenimplemented as a complementary set of pass gates which include ann-channel FET 404B and a p-channel FET 404A coupled in parallel betweena first reference voltage 412 and the node 408. The gate of FET 404Breceives state input signal 416B, and the gate of FET 404A receivesstate input signal 416A which is an inverted version of state inputsignal 416B. Input signals 416A and 416B are operated as is known in theprior art to control the switch in order to selectively charge node 408and then to selectively turn off the transistors 404A and 404B toisolate node 408 from the first reference voltage 412 so that the node408 is isolated from the first reference voltage 412 should power belost by the tag containing the state storage device 401B. In oneembodiment, FETs 404A and 404B may be implemented as long channelMOSFETs in order to reduce significantly the leakage current through theFET to make the leakage current negligible. While not shown in FIG. 5A,it will be understood that the circuit shown in FIG. 5A can optionallyinclude a discharge circuit, such as discharge circuit 410 which iscoupled to the node 408 and which is controlled by processing logic asdescribed herein.

State storage device 401B also includes a differential sense amp orsensing circuit 406A which receives the voltage at node 408 at one inputof the differential sense amp 406A and which receives another referencevoltage which is a predetermined reference voltage in one embodimentshown as reference voltage 409. In one embodiment, reference voltage 409does not equal the reference voltage 414 and is greater than the fullydischarged voltage of the capacitance circuit which includes thecapacitors 402A and 402B which are coupled in parallel between node 408and the reference voltage 414 which may be ground or V_(ss) in oneembodiment. Capacitors 402A and 402B represent one example ofcapacitance circuit 402 in FIG. 4B, and it will be appreciated that thecapacitance circuit can be implemented by a combination of one or morecapacitors in one embodiment. The capacitors may be implemented asmetal-insulator-metal capacitors or as capacitors in the same substrateas the thin oxide capacitor. In other embodiments, both the tunnelingdevice and the charge storage capacitors can be implemented asmetal-insulator-metal capacitors. FIG. 7 shows a cross-sectional view ofa semiconductor substrate which includes capacitor 402B and tunnelingdevice 402A which is one implementation of the capacitance circuit 402when it contains both a charge storage capacitor and a tunneling devicesuch as a thin oxide capacitor. As shown in FIG. 7, the tunneling device402A includes a thin gate oxide 709 which separates the gate electrode715 from the n-doped substrate region 705 which is a doped region withinthe P semiconductor substrate 703. Optional field oxide regions 707isolate the devices, such as capacitors 402A and 402B. The chargestorage capacitor 402B includes a gate 716 which is isolated from then-region 707 by a thick gate oxide 711. The n-regions 705 are coupled toV_(ss) or reference voltage 414 as shown in FIG. 7 and the gates 715 and716 are coupled to the node 408 as shown in FIG. 7.

FIG. 5B provides an example of a reference voltage generator which cangenerate a predetermined reference voltage such as the V_(ref) 409 whichis provided as an input to the differential sense amplifier 406A shownin FIG. 5A. The reference voltage generator shown in FIG. 5B can includethree MOSFETs which are shown as n-channel devices 421, 423, and 425,coupled in series as shown in FIG. 5B between reference voltage 412 andreference voltage 414. The MOSFETs are diode-coupled devices in that thegate of each MOSFET is coupled to the drain of each MOSFET as shown inFIG. 5B. It will be appreciated that the reference voltage 409 can bevaried by using different reference voltage generators in the mannershown in FIG. 5B with different numbers of MOSFETs in series and byusing different output points to obtain different reference voltages asan output from each of the different reference voltage generators. Amultiplexer can receive those different reference voltages and then theprocessing logic can select between those different reference voltagesto provide a particular reference voltage to the sense amplifier 406A.In this manner, the processing logic can select different referencevoltages which, as explained below, will result in differentpredetermined periods of time for the state storage device as will beexplained in conjunction with FIG. 6. FIG. 5C shows an example of areference voltage generator that can generate different referencevoltages that can be applied as an input to the sense amp 406A and becompared, by the sense amp 406A, to the voltage on node 408. Thereference voltage generator in FIG. 5C includes a chain of seriesconnected n-channel MOSFETs 431, 433, 435, 437, 439, and 441, with thegate of each of these MOSFETs connected, in a diode coupled manner, tothe drain of each MOSFET. There are, in this embodiment, three differentoutput taps which drive a multiplexer 443; other embodiments can usefewer or more MOSFETs with fewer or more output tags. Each output tagprovides a different V_(ref) which provides a different time period forthe persistent node. The control signal 445 is controlled by theprocessing logic (e.g. 205) in the RFID transponder to select thedesired time period; thus, the position of V_(ref) in FIG. 6 is variedto provide different time periods. The output of multiplexer 443 isinputted to the sense amp 406A.

FIG. 6 shows a graph of the discharge of capacitance circuit 402 overtime after it has been charged to a value, such as V₁ shown as 603 inthe graph 601. The capacitance circuit discharges as shown by the curve602 in the graph 601 over time. The V_(ref) 605 represents the voltageor predetermined reference voltage 409 shown in FIG. 5A. Prior artcircuits required a relatively higher current than the tunneling currentof this invention due to the difficulty of maintaining a very low bias,and thus a low current, through an FET. Prior art circuits thereforerequired relatively larger charge storage, and a detector capable ofdetecting a very low discharge point. Prior art implementations of sensecircuits for state storage devices in RFID transponders compared node408 to a fully discharged capacitance circuit; this value is shown asV_(discharge) in FIG. 6. It can be seen that V_(ref) 605 whichrepresents the reference voltage 409 shown in FIG. 5A is significantlyhigher than the V_(discharge) voltage. This allows the differentialsense amp 406A to more quickly and accurately decide the state of thenode 408. Moreover, by varying the reference voltage 409 as describedherein, different predetermined periods of time for the state storagedevice can be provided in an accurate manner using the capacitor whichcontrols the discharge over time to an accurate degree independently orsubstantially independently of temperature and semiconductor processmanufacturing variations.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications may be made thereto without departing fromthe broader spirit and scope of the invention as set forth in thefollowing claims. The specification and drawings are, accordingly, to beregarded in an illustrative sense rather than a restrictive sense.

What is claimed is:
 1. A radio frequency identification (RFID)transponder comprising: a radio frequency (RF) transceiver; processinglogic coupled to the RF transceiver; a switch coupled to a firstreference voltage; a capacitor coupled to the switch; a tunneling devicecoupled to the switch and coupled in parallel with the capacitor; adifferential sensing circuit coupled, at a first node, to the capacitorand to the tunneling device, the sensing circuit determining whether avoltage at the first node is above a predetermined reference voltage,the sensing circuit indicating a first state when the voltage at thefirst node is above the predetermined reference voltage and indicating asecond state when the voltage at the first node is below thepredetermined reference voltage, and wherein an output of the sensingcircuit is coupled to the processing logic.
 2. The RFID transponder asin claim 1 wherein the tunneling device has a tunneling current that isthrough a thin oxide of a MOS device of a CMOS process, with a thicknessbetween 10 nm and 50 nm, and the capacitor is a thick oxide MOS deviceof a CMOS process, with a oxide thickness of between 50 nm and 100 nm.3. The RFID transponder as in claim 1 wherein the area of the tunnelingdevice is smaller than the area of the capacitor.
 4. The RFIDtransponder as in claim 1 wherein the tunneling device has a tunnelingcurrent through a silicon dioxide layer with a thickness between 10 nmand 50 nm.
 5. The RFID transponder as in claim 1 wherein the majority ofthe discharge current of the switch, capacitor and the tunneling deviceis through a tunneling current through the tunneling device, and whereinthe switch includes an FET that has a long channel length to make theleakage current through the FET negligible over a temperature range from−25 degrees to +40 degrees Celsius.
 6. The RFID transponder as in claim5 wherein the tunneling device has a tunneling current through adielectric layer with a thickness between 10 nm and 50 nm.
 7. The RFIDtransponder as in claim 1 wherein the tunneling device comprises a thingate oxide which separates a conductive layer from a first substrateregion in a substrate of a semiconductor integrated circuit (IC), andwherein the capacitor comprises a thick gate oxide which separates asecond gate of the capacitor from a second substrate region in thesubstrate of the semiconductor IC, wherein the second gate and thesecond substrate region act as plates of the capacitor.
 8. The RFIDtransponder as in claim 7 wherein the tunneling device dischargesthrough a range of voltages defined by the first reference voltage atone end of the range and by the predetermined reference voltage, whichis above a ground voltage, and continues to discharge to voltages belowthe predetermined reference voltage.
 9. The RFID transponder as in claim8 further comprising: a reference voltage generator coupled to the firstreference voltage and to ground voltage and coupled to the sensingcircuit, the reference voltage generator generating the predeterminedreference voltage, and wherein the reference voltage generator does notinclude any tunneling devices.
 10. The RFID transponder as in claim 9wherein the capacitor has a greater capacitance than the tunnelingdevice.
 11. The RFID transponder as in claim 9 wherein the switch iscoupled to the processing logic and the switch comprises a field effecttransistor (FET) which selectively charges the capacitor and thetunneling device and wherein the capacitor has a greater area than thetunneling device.
 12. The RFID transponder as in claim 11 wherein themajority of the discharge current of the tunneling device is through atunneling current through the thin gate oxide, and wherein the FET has along channel length to make the leakage current through the FETnegligible over a temperature range from −25 degrees to +40 degreesCelsius.
 13. The RFID transponder as in claim 11 wherein the switch, thetunneling device and the capacitor form a state storage device thatstores a state of the RFID transponder during a loss of power in theRFID transponder.
 14. The RFID transponder as in claim 13 wherein thestate storage device is configured to store the state for apredetermined period of time which is substantially independent oftemperature.
 15. The RFID transponder as in claim 14 wherein thepredetermined period of time is more than 0.5 seconds and less than 5seconds.
 16. The RFID transponder as in claim 14 wherein thepredetermined period of time is more than 2 seconds and less than 20seconds.
 17. The RFID transponder as in claim 13 further comprising adipole antenna coupled to the RF transceiver, the dipole antennaconfigured to receive an RF signal from a reader and to transmit aresponsive RF signal to the reader.
 18. The RFID transponder as in claim17 wherein the first reference voltage is variable to store the statefor different periods of time.
 19. The RFID transponder as in claim 17wherein the FET selectively re-charges the capacitor and the tunnelingdevice while the RF transceiver receives an RF signal.
 20. The RFIDtransponder as in claim 19 wherein the majority of the discharge of thetunneling device and the capacitor is through a tunneling currentthrough the thin gate oxide and wherein the state storage device isconfigured to store the state for a predetermined period of time whichis substantially independent of temperature and wherein the state is oneof (a) an EPC RFID Class I, Generation 2 select flag or (b) EPC RFIDClass 1, Generation 2 state A or B flag.
 21. The RFID transponder as inclaim 20 further comprising: a discharge circuit coupled to theprocessing logic and coupled to the tunneling device and the capacitor,the discharge circuit configured to selectively discharge the tunnelingdevice and the capacitor in response to a signal from the processinglogic.